Thermal decomposition and transformation mechanism of mullite with the action of sodium carbonate

Highlights: • A reaction mechanism of mullite calcined with sodium carbonate is proposed. • The mechanism relies on the combination change of different oxygen coordination. • The Na₂O decomposed from Na₂CO₃ enter in mullite through filling oxygen vacancies. • The bridging oxygen bond between AlO₄ and SiO₄ was broken by the attacking of Na₂O. - Abstract: Mullite as one of the major aluminum-containing minerals in pulverized coal fly ash (PCFA) has a stable crystal structure, resulting in the difficulty in aluminum extraction from PCFA. Na₂CO₃-assisted thermal activation of PCFA is an efficient approach to activate the reactivity of mullite in PCFA. A clear understanding of the decomposition and transformation mechanism of mullite calcined with Na₂CO₃ is of great significance to clarify the activation mechanism of PCFA and the further process optimization. In this work, the thermal decomposition behavior, the phase transformation and the microstructure change were investigated in detail using thermo-gravimetric and differential scanning calorimetric (TG-DSC), X-ray diffraction (XRD) and magic angle spinning nuclear magnetic resonance (MAS-NMR). The results showed that the phase transformation process was mullite firstly converting into sodium aluminum silicate (SAS, Na_1+xAl_1+xSi_1-xO₄, 0 x 2CO₃ amount at> 800 °C. The reaction mechanism could be interpreted in terms of the combination change of different oxygen coordination. During the calcination, the Na₂O decomposed from Na₂CO₃ would enter in mullite through filling oxygen vacancies and then react with the oxygen atoms around the Al^VI, resulting in the formation of the AlO₄ in SAS. Further increase in the calcination temperature and the Na₂CO₃ amount, the transforming from mullite to SAS would be obviously enhanced. When the temperature was> 800 °C, the residual AlO₆ originated from mullite was further transformed into AlO₄ with the larger amount of Na₂CO₃. Meanwhile, the bridging oxygen bonds between AlO₄ and SiO₄ in SAS were broken by the attacking of Na₂O. Subsequently, all the obtained AlO₄ and SiO₄ as well as Na⁺ ion could be further recombined in a specific manner to transform into sodium aluminates (AlO₄-AlO₄), sodium silicates (SiO₄-SiO₄) and sodium aluminosilicates (AlO₄-SiO₄).